Methods for preparing polyether polyols, also sometimes referred to as poly(oxyalkylene) polyols, are well known in the art. Typically, such methods involve reacting a starting compound having a plurality of active hydrogen atoms with one or more alkylene oxides, such as ethylene oxide, propylene oxide, butylene oxide or mixtures of two or more of these. Suitable starting compounds include polyfunctional alcohols, generally containing 2 to 6 hydroxyl groups. Examples of such alcohols are glycol, such as diethylene glycol, dipropylene glycol, glycerol, di- and polyglycerols, pentaerythritol, trimethylolpropane, triethanolamine, sorbitol, sucrose, mannitol, etc. Usually a strong base like an alkali metal hydroxide (typically potassium hydroxide, cesium hydroxide or sodium hydroxide) is used as a catalyst in this type of reaction.
When the polymerization reaction is finished, the alkali metal has to be removed from the polymerization product (neutralization). Several ways for achieving this are known in the art. For instance, removal of the alkali metal ions by ion exchange can be applied. However, additional solvent is needed to reduce the viscosity of the polyol product sufficiently to enable an effective ion exchange. The use of such additional solvent introduces the risk of leakage, which is undesired from an environmental viewpoint, and moreover the method is expensive while the costs are even further increased by regeneration facilities in order to enable a solvent recycle. Another method to remove alkali metal from the polyether polyol product is by using absorbents like magnesium silicate. The disadvantages of this method are the high cost and the high amount of solid waste created.
U.S. Pat. No. 4,306,943 relates to a process for the preparation of polyether polyols in which the polyether polyols are neutralised by adding to the crude polyether polyol a mineral acid having a dissociation constant of greater than 10−3 at 25° C. and from 0.01 to 0.3% wt, based on the weight of crude polyether polyol, of a hydrate of a metal salt of said mineral acid for promoting the crystal growth of salt formed from the alkaline catalyst by the neutralization. Subsequently, water is distilled off from the liquid and the remaining liquid is filtered. The process of the present invention makes it obsolete to add solids such as a hydrate of a metal salt of the mineral acid used in the neutralisation. As no information is given on the amount of polyether polyol used in Control Example 2 of U.S. Pat. No. 4,306,943, it is unclear what relative amounts of water are added.
U.S. Pat. No. 4,507,475 teaches to add during neutralization 0.2 to 1.5 parts by weight of water per 100 parts by weight of crude polyether polyol and 0.3 to 1.1 moles of orthophosphoric acid per mole of basic catalyst in combination with from 0.05 to 0.5 parts by weight of magnesium silicate as adsorption agent per 100 parts by weight of crude polyether polyol. After salts and the incorporated solid additives are separated, water is removed by stripping. Contrary to what is taught in the comparative examples of U.S. Pat. No. 4,507,475, it has now been found that it is possible to prepare polyether polyols containing 20 ppm or less of sodium and potassium, without the help of adsorption agent.
DE-A-19519680 relates to a process for preparing catalytically active polyether polyols by base catalysed conversion of hydrogen containing starting compounds with alkylene oxides under conventional conditions, which process comprises either staged addition of catalyst comprising basic compounds and organic and/or inorganic alkali and/or alkaline earth salts and/or catalyst removal by a special method for crystal forming and crystal removal by addition of acids and removal of the salt crystals thereof before the crystal forming phase and/or crystal conversion phase has ended. In Example 4 according to the teaching, the salt crystals are removed when the potassium concentration of the polyether polyol is 115 ppm. The aim of the present invention is to obtain polyether polyol containing at most 20 ppm of sodium and potassium. DE-A-19519680 contains no teaching on how to obtain polyether polyols having a low sodium and/or potassium content.
A commonly used neutralisation method involves the addition of a concentrated aqueous acid solution and water to the polyether polyol reaction product in a neutralisation vessel. Typically up to 10% by weight of water based on polyether polyol is added. The acid forms salts with the alkali metal ions which salts initially dissolve in the water. The water is subsequently evaporated by heating the water/polyol mixture until salt crystals are formed and suspend in the liquid medium consisting predominantly of polyether polyol. Once the water content in the polyol has decreased to a sufficiently low level, the polyol with the salt crystals suspended therein is passed over a filter and the neutralised polyether polyol is recovered.
However, the crystals formed in the above process are relatively large and do not uniformly disperse through the polyol phase. As a result, a large amount of salt crystals adheres to the internals of the neutralisation vessel and forms a salt layer on these internals when the polyol is transferred from the neutralisation vessel to the filtration unit. After each few batches of polyol (e.g. after each second or third batch) the neutralisation vessel then needs to be washed with water to remove the salt crystals precipitated on the internals. The resulting salt solution needs to be disposed of and hence represents a further waste stream. Furthermore, due to the relatively large amount of water used, a large amount of heat is required to evaporate the water from the polyether polyol and it takes a relatively long period of time to decrease the water content to the desired low level.